Reducing the channel spacing of WDM systems is an efficient way to take full benefit of the optical amplifier bandwidth (which is around 4 THz in current amplifier systems). 100 G systems typically use a symbol rate of 28 Gbaud (PDM-QPSK, Polarization Division Multiplexing—Quadrature Phase Shift Keying) and a channel spacing of 50 GHz. Reducing the channel spacing close to the symbol rate (i.e. reducing the channel spacing close to 28 GHz) is effective in order to increase system capacity (by approximately 50%) with limited transmission reach reduction (by approximately only 10 to 20%). One key challenge in these tight channel spacing configurations is to recover the transmitter clock at the receiver. This is particularly relevant if the transmitted sequences of data symbols are relatively long (as is the case in commercial operation), because the mismatch between the clocks at the transmitter and the receiver increases with time.
WO02/31984A2 describes a method for performing multiple order amplitude modulation. US2004/101311A1 describes a method for the distribution of a synchronization signal in an optical communication system which is inherently asynchronous. U.S. Pat. No. 4,972,408 describes a method of combining and separating a low data rate digital channel with or from the high data rate digital channel of a transmission link.
WO2010/119576A1 describes a method for detecting a skew between parallel light signals generated from a serial data stream.